This invention relates to a method for phase coherent underwater acoustic communications. More particularly, the invention relates to a method for underwater acoustic communications using an improved Doppler compensation algorithm to extend the communication data packet length and reduce the number of receiver channels per fixed data and bit error rate.
Phase Coherent Underwater Acoustic Communications (ACOMMS)
The ocean presents an acoustic communication channel, which is band-limited and temporally variable. Propagation in the horizontal can be severely influenced by macro and micro multipath variability. Vertical propagation is often less severely impacted by the multipath.
Incoherent communication schemes, using for example frequency shift keying (FSK) algorithms, are used for line of sight propagation conditions in which multipath has minimal impact on the signals of interest. At long ranges, symbol rates for incoherent communications are limited by the multipath symbol interference. Additional processing (such as error encoding) is often required to remove the bit errors (due to symbol interference). The available frequency band is limited by frequency fading.
Coherent communication schemes use the available bandwidth more efficiently and provide higher data rates than the incoherent schemes for horizontal transmission of signals in a multipath environment. The state of the art systems use a (recursive) minimum least-mean-square (MLMS) approach for equalizing and updating the channel. The MLMS approach requires a certain minimum signal-to-noise ratio (SNR) at the receivers, of typically 10-15 dB. Maximum data rate and minimum bit error rate depend critically on the temporal properties of the channel impulse response function. The recursive least square (RLS) algorithm is computationally intensive and only a limited number (typically  less than 4) of channels can be supported by prototype systems for real time communications.
An algorithm for phase coherent acoustic communications is described in U.S. Pat. Nos. 5,301,167 and 5,844,951, incorporated herein by reference. The latter patent extends the algorithm from a single receiver to multiple receivers; it uses jointly a phase locking loop and channel equalizer to adaptively correct for the channel temporal variation to minimize bit errors. The communication signals are transmitted by grouping symbols into packets. Each packet begins with a short pulse (e.g., a Barker code of 13 symbols of binary phases) used for symbol synchronization and an initial estimate of the multipath arrival structure. It is followed by a data packet beginning with a training data set with known symbols to estimate the carrier frequency (shift) and train the equalizer. The equalizer is updated by estimating the symbol errors using either the known symbol as in the training data or a decision on the received symbol. The number of tap coefficients is estimated from the impulse response deduced from the probe/trigger pulse. Carrier frequency s estimated from the training data. The data are fractionally sampled, typically 2 samples per symbol, and the most popular schemes for signal modulation are binary phase shift keying (BPSK) and/or quadrature phase shift keying (QPSK) signals. Channel impulse response and equalizer update requires a minimal input signal-to-noise (SNR) ratio for minimal bit errors. Multiple receivers using spatial diversity are often required for successful communications.
Underwater Acoustic Communication Channel
The underwater acoustic communication channel is different from the RF channel in three respects: (1) the long multipath delay due to sound refraction and long duration of reverberation from the ocean boundary; (2) the severe signal fading due to time-variable transmission loss; and (3) the high Doppler spread/shift, i.e., the variability and offset of receiver frequency and phase relative to the transmitter resulting from the media and/or platform motion.
In regard to the latter effect, underwater acoustic communications often involve sources or receivers from a moving platform. The source motion induces a shift in the carrier frequency, called Doppler shift. In practice, the frequency encounters not only a shift but also a frequency broadening; the latter referred to as Doppler spread. The sources of Doppler spread are twofold: sound scattering from moving ocean surface waves which has a broad spectrum and random small jitter of platform motion. The Doppler spread determines the signal coherence time assuming that the equalizer is able to update itself within the given coherence time. Because of these differences, the various techniques for radio frequencies (RF) communications cannot be applied directly to underwater acoustic communications.
Wireless radio communications are by line of sight with some multi-paths by reflection from nearby building and structure. Multi-path interference can usually be removed by antenna beamforming using an antenna on a horizontal plane. The array configuration can be designed with element spacing and configurations based on a plane wave model: the array aperture determines the width of the beam and element spacing determines the level of the side-lobes. Multipath delays in underwater acoustic channels, however, can last tens to hundreds of milliseconds, causing inter-symbol interference to extend over tens to hundreds of symbols depending on the carrier frequency and symbol rate. Inter-symbol interference in RF channels is orders of magnitude less and thus easier to deal with. Doppler shift of carrier frequency in underwater acoustic channels is several orders of magnitude larger than that of the RF channel since the sound speed is many orders lower than the speed of light. Hence, carrier frequency identification and symbol synchronization are critical for underwater acoustic communication systems. In addition, Doppler spread is non-negligible in the underwater communication channel as sound propagates through a random ocean medium and scatters from moving surfaces. Doppler spread and frequency coherence bandwidth accordingly limit the maximum data rate of underwater acoustic communications in an ocean channel.
Because Doppler shift can be relatively large in underwater acoustic communication channels, errors in the Doppler shift estimation (the carrier frequency identification) can cause large phase errors in phase modulated symbols and mis-synchronization of the symbol sequence, as the signal bandwidth (consequently the symbol duration) also changes proportionally with the carrier frequency shift. The estimation of carrier frequency (shift) in the prior art using the training data is limited by the duration of the training data, N multiplied by T, where N is the total number of training symbols and T is the symbol duration, providing a frequency resolution of the order I/NT. This frequency resolution is usually not precise enough, resulting in a symbol synchronization error over time. As a result, the communication packet length is limited (e.g., xe2x89xa65 sec at  less than 5 kHz) beyond which the symbols are mis-aligned.
Ocean being a random medium with time varying surface boundary, acoustic signals propagating through the ocean encounter large random phase changes which must be removed or compensated for proper identification of phase modulated symbols. This was described above as the relatively large Doppler spread.
The channel equalizer can in principle track and compensate the random phase changes of each symbol. In reality, the equalizer requires tens of symbols to converge to a stable solution, as such it could not remove the random phase changes on a symbol-by-symbol basis. Thus a phase locking loop is used jointly with the channel equalizer to adaptively correct for the phase change and temporal variation of the channel impulse response. The coupling of the two operations produces a complicated inter-dependence between the phase compensation of the phase locked loop and the tap coefficients of the channel equalizer as they are being updated with the received symbol sequence. As a result, errors in Doppler shift estimation can cause a failure for the channel equalizer resulting in a high bit error rate. Spatial diversity using multiple receiver channels provides not only spatial diversity but also a minimization of the symbol phase errors by averaging the outputs of the feed forward loop over the multiple channels.
With an initial Doppler shift estimation from the training symbols, one can in principle improve the Doppler shift estimation using the decision symbols of the transmitted data, thereby improving the frequency resolution of the Doppler estimation. This scheme works well (as in RF using the pilot symbols) without the random ocean media effect. With the random ocean media effect, the phase changes of the symbols are no longer a manifestation of the (residue) Doppler shift alone, but a mixture of Doppler shift and ocean induced phase change in a complicated relationship as described above. The achievable frequency resolution is hard to estimate.
According to the invention, a method for Doppler compensation in a phase coherent underwater communications system includes the steps of: receiving a communications signal from a plurality of underwater communications channels, the signal comprising a sequence of raw data; then applying a Doppler estimation to the raw data, demodulating the raw data using the Doppler-shifted carrier frequency (fcd); low pass filtering the data; downsampling and resampling the raw data to generate a synchronized, Doppler-phase-corrected output signal; then applying the synchronized, Doppler-phase-corrected output signal to a phase locked loop and equalizer; and then comparing an estimated symbol with a decision symbol and updating a plurality of tap coefficients. The Doppler estimation may be performed by either i) applying a concurrent sinusoidal signal in a different band than the band of the communications signal; and ii) applying a Fourier transform or adaptive spectral estimation over a duration larger that a packet length to thereby obtain a Doppler-shift frequency fcd; or, by i) applying a beginning probe signal; ii) applying an ending probe signal; iii) applying a probe replica signal; iv) cross-correlating the probe replica signal with the beginning probe signal and the ending probe signal using a matched filter processor to generate a time series (e.g., a channel impulse response function); v) determine a difference of arrival time from the said cross-correlation outputs; vi) taking a ratio of the difference of arrival time to a time difference of said consecutive pulses in said communications signal to obtain a dilation/compression ratio due to the Doppler shift; and vii) obtaining the Doppler shift from the dilation/compression ratio.
The invention provides a scheme for fine-resolution average Doppler shift estimation over the communication packet, an algorithm for fast and accurate symbol synchronization, and a method for acoustic communications using a long packet, resulting an increased information data rate averaged over time.
Multiple receiver channels having a sufficient vertical aperture are usually required to minimize the bit error rate. The invention provides reduction in the minimum number of receiver channels required for fixed bit error rate and baud rate with the same input signal-to-noise ratio. Additional features and advantages of the present invention will be set forth in, or be apparent from, the detailed description of preferred embodiments which follows.